Effect of channel inclination on heat transfer and bubble dynamics during subcooled flow boiling

This study explores the influence of inclination on highly subcooled flow boiling of water in a macro-channel 10 mm high, 40 mm wide and 120 mm long. Experiments have been conducted under the following conditions. Channel inclination: 0 degrees, 30 degrees, 45 degrees, 60 degrees, 90 degrees, 120 degrees, 150 degrees; mass flux: 330, 630, 830 kg/m(2)s; heat flux in the range 300-1000 kW/m(2). Temperature recordings allow analysis of channel's heat transfer performance, while high speed video recordings provide evidence of bubbles' features. A comparative thermal and optical examination is presented for the transition region (low-heat-flux), and the nucleate boiling region (high-heat-flux). In the examined range of parameters, boiling curves are influenced more by mass flux than by inclination. Overall, operation at 60 degrees and 90 degrees yields higher heat fluxes than at other inclinations but the effect never exceeds an increase of 10% in the heat transfer coefficient compared to the horizontal case. Experimental heat transfer coefficients are in reasonable accordance with predictions of well-known empirical correlations. The role of inclination on heat transfer is explored via the analysis of bubbles' size, area density and sliding velocity. The observed bubble dynamics are in line with the measured boiling curves and heat transfer coefficients.